Nanoporous materials is one kind of nanostructured material developed in recent years. Because of its large specific surface area, Small density, adjustable structure, adjustable, and good permeability, it is widely applied in the field of separation, catalysis, Sensing, medicine, electrodes, machinery and so on.
The dealloy and template method are the mostly used to prepare porous metal. Zheng-qing Ma et al. acquired the high specific activity nanoporous silver powder by the dealloy method (reported in CN101391304A), the steps are: smelting the magnesium in the argon atmosphere, add the metallic silver when magnesium melt completely; then casting the silver-magnesium alloy; then mechanical crushing, ball milling, classification the casting product when it is cold. Then the Ag—Mg alloy powder was corroded by mixed acid to get to the size requirement, at last the porous silver powder was acquired after the treatment of washing, filtering and drying. Due to high temperature and argon atmosphere are needed in the smelting process when preparing the raw material, it is difficult for the equipment to acquire these high requirement. At the same time, the nitric acid and the sulfuric acid are always used as corrosion electrolyte in the electrochemical dissolution process. Therefore, the dealloy method is harmful to the operators and the environment. Jin R H and Yuan J J have (Journal of Materials Chemistry 15 (2005) 4513) prepared the macroporous silver powder by using poly (ethyleneimine) as template. The complex process and high cost make the template method hard to industrialization, also the pore structure of silver prepared by template method is single and hard to control.
Adzic team found that Cu monolayer can be formed on the noble metal particle surface through Cu-UPD, then Pt monolayer catalyst can be acquired through replacement reaction with Pt2+, the highly dispersed Pt greatly enhanced the ORR mass catalytic activity of this kind of catalyst. However, precious metals, such as Au and Pd are always used as the core of this kind of catalyst, which makes the high cost of it. Compare with Pt, Pd and Au, Ag shows the advantage of large quantity and low cost, but short of research. Porous silver show great application prospect due to its high surface area and the good mass transfer ability, however, silver is easy to be corrode in the acid which limit its application in acidic environment. By replacing with noble metal (Pt, Pd), we can get noble metal shell-porous silver core material, which can be used in acidic environment. But this kind of research has not been reported.
The silver causes wide attention of the researchers because of its high activity and good stability in alkaline medium and the outstanding advantage of the methanol permeability resistance. The product of Ag/C catalyst in the field of alkaline anion exchange membrane fuel cell does already exist, while the ORR over potential of Ag/C was about 100-150 mV higher than Pt/C catalyst (reported in J. Electrochem. Soc. 152 (2005) D117). By combine Ag with Co3O4, the ORR catalytic activity can be enhanced (reported in Electrochem. Commun. 31 (2013) 108); tang et al. prepared Ag/Mn3O4/C catalyst using one step method (reported in Appl. Catal. B: Environ. 104 (2011) 337), which reduce the gap of ORR over potential to Pt/C to 31 mV. That is to say, by combine silver with the oxide of manganese, cobalt et al., the ORR catalytic activity can be enhanced. Linic et al. (Nature Chem. 6 (2014) 828) reported that the catalytic activity of Ag can be enhanced by form AgCo alloy through quantum chemistry calculation. They also prepared the catalyst with AgCo alloy surface, which shows enhanced ORR catalytic activity compared with Ag, but the ORR activity compared with the Pt/C catalyst still has certain gap.